From the propagation characteristics of the electromagnetic wave, we know that when the user switches the frequency or moves in a short time, its surrounding environment would not change much, which makes the channels at different spaces or different frequencies to have certain mapping relationship. Many efforts have been made to explore such mapping, and some effective mapping models, e.g., the one between the uplink and downlink channels over the massive MIMO systems, have been established.
Nevertheless, channel mappings, like the one between the sub-6 and mmWave bands or the one among different distributed antennas, are difficult to describe mathematically.
In [1]}, the authors proved that the large-scale antenna arrays are able to assure the bijectiveness condition, which makes the channel mapping along the frequency and space dimensions unique. Then, the authors utilized the DL approach to successively approximate such channel mapping.
The key idea of DL-based channel extrapolation is illustrated in Fig. REF .
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